(a) Field of the Invention
The present invention relates to a photographic lens system for popular cameras and, more particularly, to a vari-focal photographic lens system of lens unit exchange type to be used with popular cameras.
(b) Description of the Prior Art
Known vari-focal photographic lens systems of lens group exchange type to be used with popular cameras are disclosed, for example, in Japanese published unexamined patent applications No. 132305/81 and No. 143407/81. Each of the lens systems disclosed in the above-mentioned Japanese published unexamined patent applications comprises three lens groups, i.e., a first lens unit having positive refractive power, a second lens unit having negative refractive power, and a third lens unit having positive refractive power, and is arranged that the lens unit having negative refractive power is exchanged for a different lens unit and, at the same time, the first lens unit having positive refractive power is moved along the optical axis so as to change the focal length of the lens system and, at the same time, to fix the image surface.
In the above-mentioned type of lens systems, a lens unit having positive refractive power is arranged on the image side and, therefore, the power distribution of said lens systems is different from the power distribution of telephoto type lens systems in which a negative lens component is arranged on the image side. This means that the lens composition of said lens systems is disadvantageous for making the overall length of the lens system short.